[PowerPC64] fix crash when using machdep.moea64_bpvo_pool_size tunable

Summary:
This fixes kernel crashing when tunable "machdep.moea64_bpvo_pool_size" is
set to a value higher then 327680 (default value). Function
moea64_mid_bootstrap() relies on moea64_bpvo_pool_size, but at time of the
use the variable wan't yet updated with the new value provided by user.

Problem was detected after trying to use a VM with 64GB of RAM, and default
moea64_bpvo_pool_size is insufficient (kernel boot used more than 470000) .
I think default value must be discussed to address this use case, or find a
way to calculate pool size automatically based on amount of memory detected.

Test Plan: Tested on QEMU VM with 64GB of RAM using "set
machdep.moea64_bpvo_pool_size=655360" on loader prompt

Submitted by: Alfredo Dal'Ava Júnior (alfredo.junior_eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D23233